The present invention relates to the efficient and selective precipitation of manganese, from magnesium-containing solutions. More particularly, the invention relates to the removal of manganese from, for example, laterite ore waste solutions which are substantially barren of one or more of cobalt, nickel, copper and zinc, but which contain magnesium, manganese and aluminum.
Mining and milling operations generate various types of toxic, metal containing effluents which require treatment prior to discharge to the environment. These effluents include, for example, acid mine drainage, mill tailings excess decant water, seepages, and acidic process waste streams. The most common of these is acid mine drainage, characterized by acidity (sulphuric) and metals, which may include aluminum, cadmium, chromium, cobalt, copper, iron, lead, magnesium, manganese, nickel, zinc and others.
The processing of nickeliferous lateritic ores by sulphuric acid leaching has gained considerable interest in recent years, with three commercial plants based on high temperature acid leaching coming on stream in Western Australia in the late 1990s. A number of similar operations are at various stages of development, throughout the world. The process generates acidic product liquors, containing nickel and cobalt, as well as most of the afore mentioned metals, as impurities. In addition, the product liquors contain significantly higher concentrations of manganese and magnesium, particularly relative to their concentrations usually encountered in acid mine drainage.
Various methods are used and have been proposed for recovery of the nickel and cobalt from such leach liquors. These fall into three general categories, including precipitation as sulphides, precipitation as hydroxides (to produce intermediates for subsequent refining) and direct solvent extraction. Most of these options require removal of at least some of the metal impurities prior to recovery of the nickel and cobalt. Recovery of the latter by sulphide precipitation requires prior neutralization of the acid but, because sulphide precipitation is relatively selective for the base metals, may require little or no prior removal of metal impurities. Recovery by the other alternatives also requires prior neutralization of the acid and removal and/or reduction of some of the metal impurities, such as aluminum, chromium and iron before recovery of the nickel and cobalt. Regardless of the method of recovery of the valuable metals, the barren or waste solution will still contain varying and variable concentrations of toxic impurities. In addition, some of the recovery alternatives for the nickel and cobalt may involve a co-extraction or co-precipitation of some of the metal impurities, which are removed or rejected at a later stage in the process, as a secondary waste or effluent stream which, in most instances, is recombined with the major effluent stream.
The most common and effective way of dealing with acidic metal containing effluents prior to discharge of the treated water to the local waterways is neutralization of the acid and precipitation of the dissolved metals as hydroxides, using suitable alkaline reagents. Lime is most commonly used as the neutralizing/precipitating reagent, because of its high reactivity, availability, and relatively low cost. Alternatively, the sequential use of limestone, to pH 5 to 6, in a first stage, to precipitate the bulk of the aluminum, chromium and iron, followed by lime, to pH 8 to 10, to precipitate the remaining metals, may be preferred. Depending on the environmental regulations, which are usually site-specific and may vary considerably from site to site, the treatment with the combination of limestone and lime, or by lime alone, may be adequate for meeting the requirements for the treated water. Air is frequently used during the neutralization, to oxidize ferrous iron to ferric and, when using limestone in the first stage of neutralization, to remove the generated carbon dioxide prior to the addition of lime.
A treatment to a pH range of 7 to 8 is generally sufficient for removal of most of the impurities to acceptable levels. One exception, however, is manganese. According to one reference (N. Kuyucak, xe2x80x9cConventional and New Methods for Treating Acid Mine Drainage,xe2x80x9d proceedings of CAMI ""95 Conference, Oct. 22 to 25, 1995, Montreal, Quebec), removal of the manganese requires strong oxidation followed by liming at pH greater than 10. Many of the effluents also contain appreciable concentrations of magnesium, however, and neutralization to such high pH levels also results in precipitation of the magnesium, which is generally not categorized as toxic nor whose removal is required. For example, McLaughlin et al, in a paper entitled xe2x80x9cA Comparison of Selected Acid Mine Drainage Treatment Processesxe2x80x9d (Preprint 96-145, SME Annual Meeting, Phoenix, Ariz., March 11 to 14, 1996) state that to remove manganese to low levels in a reasonable period of time (pH approximately 10.5), a significant portion of the magnesium present will also precipitate. This is further illustrated in a paper by Feng et al, entitled xe2x80x9cTreatment of Acid Mine Water by Use of Heavy Metal Precipitation and Ion Exchangexe2x80x9d (Minerals Engineering, Vol. 15, No. 6, pp. 623 to 642, 2000). In their work, acid mine water, containing a wide range of metals, including 113 ppm Mn and 359 ppm Mg, was treated with lime to precipitate the metals. By pH 9.1, the Mn had been precipitated to 15.7 ppm, without appreciable co-precipitation of Mg. Further liming, to pH 10.1 had lowered the Mn concentration to 2.6 ppm, but with co-precipitation of about 60% of the Mg, to 143 ppm. By the time the Mn had been removed to 1.1 ppm, the co-precipitation of the Mg had been even more complete, to 0.5 ppm. Thus, additional reagent had to be added to precipitate the Mg with the Mn to get the Mn concentration to the required levels. In instances where removal of magnesium is not required and/or where more effective and selective removal of the Mn is required, the alternative treatments which have been adopted have included the use of strong chemical oxidants or the use of sulphiding reagents.
Laterite ore leach solutions and the resultant waste solutions, after recovery of the nickel and cobalt, are characterized by high concentrations of most impurity metals-relative to typical concentrations in most acid mine waters and other mining and milling wastes. For example, the barren solutions may contain from 0.5 to 5 g/L Mn, and from 3 to 50 g/L Mg, depending on the ore type treated, and the extent of solution recycling within the processing plant. Environmental regulations for plant effluents vary considerably, depending on the nature of the receiving waters, location and a number of other factors. In many locations, the discharge of magnesium containing solutions is allowed, whereas prior removal of manganese to trace levels is required. The allowable manganese level may range from several tens of mg/l, to 1 mg/l or less, depending on the site. Although, as noted earlier, the selective removal of manganese might be accomplished by the use of strong oxidants or of sulphiding reagents, the relatively high concentrations and quantities of manganese in laterite leach effluents would tend to make these alternatives economically unattractive or prohibitive. The ability to effect an extensive and selective removal of the manganese, to trace levels, in the presence of appreciable concentrations of magnesium with inexpensive reagents such as lime would, therefore, be highly desirable.
The present invention provides a process for selective removal of manganese from acidic waste solutions which are preferably substantially barren of one or more of cobalt, nickel, copper and zinc, but which contain manganese, magnesium and aluminum (and possibly other metals, such as iron and chromium), without unnecessary co-precipitation of magnesium. The process not only enables the use of inexpensive reagents, such as lime, it also requires a reduced amount of reagent to precipitate manganese effectively.
Conventional precipitation of environmentally sensitive metals such as Al, As, Cd, Cr, Co, Cu, Fe, Ni, Zn and, especially, Mn, from effluent streams, using lime or sequential limestone and lime is technically feasible. However, as noted above, in solutions also containing magnesium, the removal of the manganese to trace levels typically results in the unavoidable co-precipitation of a significant portion of the magnesium, thereby increasing lime requirements to between five and ten times those required for the manganese alone. When metals are removed from a waste stream, the precipitated solids resulting from treatment to remove the metals, such as manganese, become solid waste that must also be handled. Thus, any unnecessarily precipitated magnesium not only increases the amount of reagent required for manganese removal, it also adds to the quantity of waste solids. Also, since manganese is often removed from waste streams for the purpose of decontaminating the waste solution put into the environment, rather than for the recovery of manganese, the use of more expensive treatment methods, such as the use of chemical oxidants or sulphiding reagents, is not economical. Thus, the inventors recognized the need for a selective method of manganese precipitation using inexpensive reagents.
It has been discovered that the prior precipitation and physical removal of a majority of aluminum enables an efficient and highly selective precipitation of the manganese, with minimal co-precipitation of the magnesium, thereby saving appreciably on the lime requirements.
Based on published information on the solubility of metal hydroxides (Log Ks for Mnxe2x80x9412.7, Mgxe2x80x9411.3) it was believed that there should be some scope for the selective precipitation of manganese, relative to magnesium. Specifically, Monhemius (xe2x80x9cPrecipitation Diagrams for Metal Hydroxides, Sulphides, Arsenates and Phosphates,xe2x80x9d Trans. IMM, December 1977, C202 to C206.) shows that manganese should, theoretically, precipitate at a lower pH than magnesium and thus, precipitation at this pH should be selective for manganese. However, in practice, and as demonstrated herein, selective precipitation does not occur. Without being limited to such, it is believed that magnesium precipitates with manganese when a highly reactive reagent is used, such as lime, partly because the reagent does not distinguish between the two metals, given that they become insoluble at relatively close pH levels. It is possible that areas of localized pH are formed that are higher than the level at which magnesium becomes insoluble and, as such, it is difficult to selectively precipitate manganese without magnesium. Thus, recognizing the difficulties, the inventors performed several tests to find a method of either effecting selectivity, or of reducing the reagent requirements to remove manganese. Several series of tests were conducted to evaluate the possibility of selective precipitation of manganese and its limitations. The tests were conducted on a wide range of solutions, most of them generated in continuous pilot plant leach operations on a variety of laterite ore feeds, after nickel and cobalt recovery from the leach solutions by sulphide precipitation.
The barren solutions, used in the tests demonstrated in Examples 1 to 3 and 5 to 7, contained 0.6 to 5.9 g/l Al, 0.1 to 0.6 g/l Cr, 0.1 to 1.8 g/l Fe, 3.3 to 19.6 g/l Mg, and 1.4 to 3.5 g/l Mn, as the major metal ion components. The neutralization and metal hydroxide precipitation tests were conducted with lime and limestone/lime combinations as the alkaline reagent, in both batch and continuous mode, using both single point and staged addition of the alkaline reagents, and on both barren solutions and on slurries of barren leach solids and barren solution.
The results of the tests using usual prior art neutralization methods, as demonstrated in Examples 1 to 3, showed that treatment with lime, or the sequential treatment with limestone and lime was effective in precipitating the manganese to  less than 5 mg/l, even from the high magnesium-containing solutions and systems. However, this was at the expense of the co-precipitation of a significant proportion of the magnesium, such that the lime requirements for the removal of the manganese to trace levels were from 5 to 10 times the stoichiometric amount for manganese alone. For example, removal of the manganese from a solution containing 1.58 g/l Mn and 17.2 g/l Mg, to  less than 10 mg/l Mn, as described in Example 1, resulted in the co-precipitation of almost 7 g/l Mg. Thus Examples 1 to 3 confirm the conclusions of the prior art, which indicated that complete removal of the manganese as a hydroxide or hydrated oxide resulted in significant precipitation of the magnesium.
A more extensive study was then undertaken, using both synthetic and actual barren solutions produced in the acid leaching of several laterite ores, to examine possible means of improving the selectivity of manganese precipitation relative to magnesium, as a hydroxide or hydrated oxide. These studies, as described in Examples 4 to 11, established, unexpectedly, that the presence of aluminum in the precipitation system adversely affected the ability to effect the selective precipitation of manganese. It was found that prior precipitation and physical removal of a majority of the precipitated aluminum species (aluminum-containing solids) enabled subsequent complete and highly selective precipitation of the manganese in the presence of magnesium. Without being bound by such, it appears that, in such systems, the addition of the lime reagent results in the precipitation of a mixture of magnesium and manganese hydroxides but, surprisingly, with sufficient retention time, the magnesium hydroxide exchanges for remaining dissolved manganese, with precipitation of the manganese, and re-dissolution of the magnesium. In the presence of precipitated aluminum, however, it appears that the aluminum, possibly due to its amphoteric nature, in some manner inhibits reaction of the precipitated magnesium hydroxide with the remaining soluble manganese, and necessitates the addition of large and excessive amounts of lime, resulting in precipitation of both the manganese and appreciable magnesium. These effects are illustrated in the provided examples.
Broadly stated, the invention provides a process for selectively precipitating and removing manganese from an acidic solution, preferably substantially barren of one or more of cobalt, nickel, copper and zinc, but containing manganese, magnesium, and aluminum, comprising:
a) adding a first alkaline reagent to neutralize the acidic solutions and to precipitate a majority of the aluminum as aluminum-containing solids, without precipitating a substantial amount of the magnesium;
b) removing the precipitated aluminum-containing solids to create an aluminum-depleted solution;
c) adding a second alkaline reagent to the aluminum-depleted solution and aerating for a sufficient retention time to preferentially precipitate a majority of the manganese as manganese-containing solids; and
d) removing the precipitated manganese-containing solids.
By xe2x80x9cbarrenxe2x80x9d, as used herein and in the claims, is meant that certain metals (usually one or more of cobalt, nickel, copper and zinc) have already been removed from a solution, down to trace or non-useful levels. Generally, these levels will be less than about 50 mg/l, more usually in the range of 5-30 mg/l.
By xe2x80x9cmajorityxe2x80x9d, as used herein and in the claims is meant more than 50%. More preferably, for aluminum, an amount greater than 90% is removed, and for manganese, an amount greater than 95% is removed.
The tests were performed with the acidic solutions over the general temperature range of 23 to 85xc2x0 C., to simulate expected application temperatures. In laterite pressure leaching processes barren solutions are generally treated in the first stage at 80xc2x0 C. or higher. When the process combines the barren solution with washed leach residue solids before final neutralization, the neutralization usually occurs at a temperature closer to ambient. The process was tested at 40xc2x0 C. to simulate the usual solution temperature in a warm southern climate. However, it is anticipated that the process is applicable at a much broader range of temperatures, such as 23 to 95xc2x0 C., and with 40 to 80xc2x0 C. being the more preferred range.
The preferred embodiment of the invention is a process used to remove metals from an acidic waste solution from, for example a laterite ore leaching process, wherein the acidic waste solution is barren of one or more of cobalt, nickel, copper-and zinc, most preferably barren of cobalt and nickel, but contains magnesium, manganese and aluminum, and possibly other metals such as iron and chromium. Generally, the acidic waste stream to be treated is in the temperature range from 40 to 80xc2x0 C.
Generally, tie waste stream to be treated includes metals in the following ranges: magnesium 3.0 to 50 g/l; manganese 0.5 to 5.0 g/l, aluminum 0.5 to 8.0 g/l; iron 0 to 5.0 g/l; and chromium 0 to 1.5 g/l.
In the process of the invention, a first alkaline reagent is added to neutralize the acidic solution and to precipitate a majority of the aluminum as aluminum-containing solids. These solids are then removed to create an aluminum-depleted solution. Next, a second alkaline reagent is added to the aluminum-depleted solution, with aeration for a sufficient retention time to preferentially precipitate a majority of the manganese as manganese-containing solids. These manganese-containing solids are then removed from the solution.
The pH level achieved with the addition of the first alkaline reagent (sometimes referred to herein as acid neutralization or the first stage of metal precipitation) should be in the range of 3.5 to 7.0, more preferably in the range of 4.0 to 5.5, in order to precipitate a majority of the aluminum as aluminum-containing solids without precipitating a substantial amount of the magnesium. At these pH ranges, magnesium remains largely in solution such that co-precipitation of magnesium is not substantial (preferably less than 10% and more preferably less than 2% by weight). These solids are then removed from the solution by any known solids/liquid separation technique such as filtration. The process is capable of reducing the aluminum to less than 0.5 g/l, more preferably to less than about 0.1 g/l.
After removal of the precipitated solids from the treated solution, a second alkaline reagent is added to achieve a pH level, for the second stage of metals precipitation, in the range of 7.0 to 9.0, more preferably in tie range of 7.5 to 8.5.
Preferred first alkaline reagents are reagents which provide one or more of an oxide, hydroxide or carbonate of one or both of calcium or magnesium. Table A below shows typical reagents to supply these species, with one or both of limestone and lime being most preferred.
Preferred second alkaline reagents are those which provide one or more of an oxide or hydroxide of one or both of calcium or magnesium, or sodium carbonate or sodium hydroxide. Table A below shows typical reagents to supply these species, with lime being most preferred.
The first and second alkaline reagents may be added as solutions, depending in part on their solubilities, as aqueous slurries or other streams containing these reagents, or they may be added as powdered reagents. It should be understood that this first and second alkaline reagent list below is meant to include powdered reagents, as well as aqueous slurries, solutions or other streams which contain one or more of the oxide, hydroxide or carbonate species listed below.
The process of die present invention has been demonstrated to be operative at atmospheric pressure, and can be conducted at temperatures in the range of 23 to 95xc2x0 C., but is more preferably conducted at temperatures in the range of 40 to 80xc2x0 C.
The process of this invention has the major advantage of accomplishing the selective precipitation of manganese (over magnesium) to remove more of die manganese, with less consumption of the total of the first and second alkaline reagents, than would be needed without the aluminum precipitation and removal step.
In a preferred step of the process, tie first step of adding die first alkaline reagent is accomplished with aeration or gas sparging to dispel any formed carbon dioxide and/or to oxidize any ferrous iron. This step is most beneficial when the first alkaline reagent is a carbonate reagent. If it is desired to oxidize the ferrous iron, aeration is preferred with air or other oxygen-containing gases, rather than sparging with other gases.
Alternatively, when acidic solutions contain ferrous iron, it may be beneficial to allow at least a portion of the ferrous iron to proceed through to the second metal precipitation step to be co-precipitated with the manganese-containing solids, since there is evidence that the ferrous iron appears to improve the subsequent precipitation of the manganese.
In the examples that follow, particularly Example 6, further preferred operating conditions have been discovered. A test series on aluminum-free manganese-magnesium sulphate solution showed that the addition of lime initially precipitated a mixed Mn-Mg oxide aid hydroxide, some of which was identified by X-ray diffraction analysis as MgMn2O4 (MgOxc2x7Mn2O3). With extended time and aeration, the precipitated magnesium exchanged with die remaining soluble manganese, removing it to trace levels. Higher temperatures promoted more rapid and more complete precipitation. With 150% of the stoichiometric requirement of lime for the Mn (initially 1.9 g/L), the manganese was removed to about 1.5 mg/L, within 2 h at both 80xc2x0 and 50xc2x0 C., whereas at 27xc2x0 C., the Mn was precipitated only to 480 mg/L, by 2 h, and 110 mg/L, by 6 h. With 200% of stoichiometric lime, at 27xc2x0 C., the Mn was removed to 210 and 26 mg/L by 2 and 6 h, respectively. The addition of an oxidant, preferably air, was important at all temperatures, as without it, there was no further precipitation of manganese over that in the initial Mu-Mg oxide or hydroxide just after the lime addition.
A test series with aluminum-containing solutions, with limestone precipitation of the aluminum followed by the lime precipitation of the manganese without an intermediate solids/liquid separation stage to remove the precipitated aluminum-containing solids, established a strong dependence of lime requirements for complete manganese removal on the initial aluminum concentration in the feed solutions. With 1, 3 and 5 g/l Al in the feed, lime requirements for effective removal of die manganese increased to about 415, 750 and 1,230% of the stoichiometric requirement based on the concentration of manganese, as increasingly more of the magnesium was co-precipitated, but was not effective in removing the remaining manganese. At the point at which the manganese had been precipitated to  less than 5 mg/L, the Mg:Al molar ratio in die corresponding solids was almost 2:1. This suggested that much of the magnesium which was initially precipitated had been tied up as a complex by the aluminum.
Manganese precipitation tests with lime were also conducted on a slurry comprising aluminum-free solution and aluminum-containing washed laterite leach residue, at 50 and 80xc2x0 C. The aluminum in die solids, present predominantly as alunite which had been precipitated under autoclave conditions, was only slightly deleterious in the precipitation of the manganese, the effect being greater at the higher temperature. This would suggest decomposition of some of the alunite at 80xc2x0 C. (more so than at 50xc2x0 C.), but the interference was considerably less than that by an equivalent amount of atmospherically-precipitated aluminum.
Manganese precipitation tests were also conducted in the presence of chromium and iron; neither of these metals had the interfering effect displayed by aluminum In fact, iron, particularly ferrous iron, proved beneficial. When processing actual waste solutions, therefore, it may be advisable to retain some of the ferrous iron in solution during die limestone precipitation and removal of aluminum, to enhance the subsequent selective precipitation of the manganese.